The distance from the edge of a door to the center of rotation of the door knob or lever is called the "backset". In the United States, doors are commonly available in two backsets, 23/8" and 23/4". Until recently, it was customary for hardware companies to produce locks in both backsets, and retailers would stock both types. Now, however, large retailers wish to limit the number of types of each product they have to stock, so they are demanding that their vendors supply locks that can be easily adjusted to fit either backset. The current invention provides a method for doing so.
The latch of the instant invention is of the type commonly referred to as a "tubular latch", meaning that the operating mechanism is contained within a case that can be inserted axially into a hole (usually 1" in diameter) drilled in the edge of the door. In its most basic form, it consists of a spring loaded bolt that extends from the edge of the door and engages a hole in the strike plate attached to the door jamb. To open the door, the bolt is withdrawn by turning a handle that is attached to a cam that rotates in the latch case and engages detents on a drawbar that is attached to the bolt head. Since the backset is the distance from the edge of the door to the center of rotation of the knob and, by extension, the cam, the backset can be adjusted either by moving the front of the case and the bolt head to either of two positions relative to the cam center line, or by providing two sets of detents and a means of moving the cam so that it engages either one or the other. Both types are currently in use. The invention described here belongs to the latter group.
If the cams are to engage the detents in order to operate the latch, but be able to move from one set of detents to the other to adjust the backset, some means must be provided to disengage the cam from the detent at the appropriate time. The simplest way to accomplish this is to make the arms of the cam narrower than the distance between the detents when the cam is in its rest position. The cam can then be moved to either of its two operating positions without interference. This method works, but in operation a certain amount of rotation is necessary before the cam begins to engage the detent, resulting in an undesirably large amount of total rotation. The instant invention remedies this problem.